The present study investigates the groundwater solute chemistry, hydrogeochemical behavior of arsenic (As) and the assessment of health risks through ingestion and dermal contact pathways to the adults and children of the Bahraich district, falling under the middle Gangetic plain, Ghaghara river sub-basin, Terai region of India. A Monte Carlo simulation and a sensitivity analysis were also performed to quantify the uncertainties and impact of various input variables in risk calculations, respectively. Concentrations major ions (Ca2+, Mg2+, Na+, K+, SO42−, PO43− NO3−, HCO3−, F− and Cl−), dissolved organic carbon (DOC), and trace metals (As, Fe, and Mn) as well as physical parameters (EC, pH, and Eh) were measured on the collected groundwater and river water samples. The concentration of As in the groundwater samples was found in range between 0.64 μg/L to 104 μg/L and ∼37% of the samples had As>10 μg/L (WHO and BIS acceptable limit of 10 μg/L for drinking water). River water samples also displayed high As concentrations (mean of 14 μg/L) with ∼73% of samples having As>10 μg/L, which could be attributed to infiltration from As-enriched sallow groundwater of the nearby regions. The groundwater of the study area was predominantly of Ca2+-Mg2+-HCO3- type. Bivariate weathering plots showed groundwater was influenced by silicate weathering and carbonate dissolution, along with the ion exchange and reverse ion exchange processes, with a minor contribution from evaporate dissolution. Most of the groundwater samples (n = 57) were anoxic and had low SO42− and NO3−, and high Mn, Fe and DOC concentrations. The observed values and correlations between various measured parameters, including groundwater As concentrations and saturation indices calculations, indicated the reductive dissolution of iron oxyhydroxide as the major process for As mobilization in the study area. The results of total non-carcinogenic risk (HI) estimated by the deterministic and probabilistic techniques were nearly identical for both adults and children, but an overestimation was observed in the case of carcinogenic risk calculated by the deterministic approach. Health risk results also showed that children were more susceptible to non-carcinogenic risk, whereas adults were at a higher risk of cancer in the study area. Sensitivity analysis indicated that the concentration of As in groundwater and exposure duration (ED) were the most effective variables for non-carcinogenic (HI) and carcinogenic (TILCR) risk estimation in both adults and children.